1. Technical Field
The invention relates to a method for operating a mass spectrometer with an ion source and an analyzer, a gas being admitted into the mass spectrometer, ionized at the ion source and analyzed in the analyzer. In addition, the invention relates to a mass spectrometer with an ion source and an analyzer for the analysis of gases.
2. Prior Art
Static mass spectrometers are used when extreme sensitivity is important. The analysis of noble gases (He, Ne, Ar, Kr, Xe) in rocks, meteorites etc., but also in water samples, is typically concerned. Argon is particularly important. Sometimes the term “noble-gas mass spectrometer” is therefore customary. However, such mass spectrometers can also be used for the analysis of gases such as CO2 or N2.
The static mass spectrometer comprises an ion source, an analyzer, in particular, but not necessarily, with a magnetic sector, and a collector, for instance a Faraday cup, or secondary electron multipliers (SEM). Of course, the static mass spectrometer also has devices for producing a high vacuum, that is a pump with a corresponding system of lines. During a measurement, the gas to be analyzed is not pumped out of the static mass spectrometer. Rather, the mass spectrometer is disconnected from the pump after evacuation and then the gas to be analyzed (sample gas) is admitted and distributes itself in the mass spectrometer (ion source, analyzer and collector region). The bonding of reactive gases by so-called getters is known.
In conventional static mass spectrometers, the gases to be analyzed are ionized in the ion source by electron impact. In this case, the gas particles are bombarded with electrons of high kinetic energy (for example 40 to 150 eV). Ionization by laser bombardment is likewise possible.
The known ionization of the gas by electron impact is not particularly effective. The density of the particles to be ionized is relatively low in the gas phase. This results in an only low ion current. This cannot be compensated by increasing the amount of the gas to be analyzed, since the amounts of gas available are generally minimal. The highest possible number of ions is also desirable for reasons of ion counting statistics—more accurate measurement results with an increasing number of ions. Conventional static mass spectrometers operate with typical sensitivities of approximately 1 mA/Torr of gas pressure. A higher value is aimed for. One disadvantage of the conventional mass spectrometers in connection with electron impact ionization is also the high energy width of the ions generated. This limits the achievable mass resolution, in particular in the case of single-focusing mass spectrometers.